As fuel cells using a polymer electrolyte membrane can be reduced in size and weight and have high power generation efficiency and energy density, fuel cells are expected to serve as a power supply for mobile devices using methanol, hydrogen, or the like as a fuel, a power supply for household cogeneration systems, and a power supply for fuel cell vehicles. In such fuel cells, a polymer electrolyte membrane, an electrode catalyst, a gas diffusion electrode, a membrane-electrode assembly, and the like are the major element technology. Development of a polymer electrolyte membrane having excellent properties for use in fuel cells is one of the most important component technologies.
In a solid polymer fuel cell, an electrolyte membrane acts as an “electrolyte” for conducting hydrogen ions (proton), and also acts as a “separator” for inhibiting hydrogen and methanol, both of which are fuels, from coming into direct contact with oxygen. Such a polymer electrolyte membrane is required to have a large ion exchange capacity. Chemical stability is also required to be tolerant of a long period of use; a polymer electrolyte membrane is especially required to exhibit a high proton conductivity and to be stable at a temperature of 80° C. or higher, which is an operating temperature of a cell, in any moisture state from a dry state to a flooding state. To serve as a separator, a polymer electrolyte membrane is required to have excellent mechanical strength, excellent dimensional stability, low hydrogen permeability, low methanol permeability, and low oxygen permeability.
Perfluorosulfonic acid membranes “Nafion (registered trademark of DuPont)” developed by DuPont and other membranes have been commonly employed as an electrolyte membrane for use in solid polymer fuel cells. Although the conventional fluorine-containing polymer electrolyte membranes such as Nafion® have excellent chemical durability and stability, they have a low ion-exchange capacity of about 1 mmol/g. Furthermore, when hydrogen is used as a fuel, cross-over of hydrogen and oxygen occurs. The membranes also have a disadvantage that mechanical properties of the membranes are significantly decreased at a temperature of above 100° C., which is a required operational condition of a power supply for use in vehicles. Further, as the production of a fluorine resin-containing polymer electrolyte membrane starts with the synthesis of a monomer, the number of production steps increases, causing the membrane to be expensive. This is a major obstacle to practical use as a power supply for household cogeneration systems or a power supply for fuel cell vehicles.
Thus, low-cost polymer electrolyte membranes to replace the fluorine-containing polymer electrolyte membranes have been actively developed. For example, in the development of a low-cost polymer electrolyte membrane, attempts have been made to produce an electrolyte membrane (fluorine-containing graft electrolyte membrane) for use in solid polymer fuel cells by graft polymerization of a fluorine-containing polymer membrane substrate, such as polytetrafluoroethylene, polyvinylidene fluoride, and an ethylene-tetrafluoroethylene copolymer, with a styrene monomer to introduce the monomer into the substrate, followed by sulfonation of the monomer (Patent Publications 1 and 2). However, since a fluorine-containing polymer membrane substrate has a low glass transition temperature, it has disadvantages that the mechanical strength of the membrane is significantly decreased at a high temperature of 100° C. or higher, and that it causes cross-over of hydrogen and oxygen used as fuels. Thus, attempts have been made to produce an electrolyte membrane (aromatic graft electrolyte membrane) by the graft polymerization and sulfonation using as a polymer substrate a wholly aromatic engineering plastic having excellent mechanical properties and fuel barrier properties at high temperatures (Patent Publication 3).
However, it is pointed out that during operation at a high temperature in the presence of water, polystyrene graft chains are decomposed to cause a deterioration involving a decrease in an ion exchange capacity of a membrane (Patent Publication 4). To inhibit the decomposition of graft chains observed in polystyrene sulfonic acid, attempts have been made to produce a polymer electrolyte membrane (alkyl type graft electrolyte membrane) comprising an alkyl sulfonic acid in the graft chains and having high ion conductivity, low fuel permeability, excellent resistance to hot water, and oxidation resistance by radiation-induced graft polymerization of monomers having an acrylic acid derivative or a vinyl ketone derivative as a skeleton, and then introduction of sulfonic acid groups into graft chains of the resulting polymer (Patent Publication 5).